AN INTERGLACIAL VALLEY IN ILLINOIS. 



In this paper it is our purpose to trace the complex history of 

 the present Embarras River valley, in southeastern Illinois, 

 which lies partly on the newer Wisconsin drift and partly on the 

 older Illinoian drift, crossing the terminal moraine of the former 

 about midway of its course. North of the moraine the Embarras 

 Valley is entirely postglacial, with little relation to any former 

 valley. South of the moraine the youthful postglacial valley, as 

 far as it extends, lies within a wider and more mature intero^lacial 

 valley, which, in turn, usually lies within a still more mature 

 preglacial rock valley. 



There is evidence in Illinois and vicinity of several advances 

 and retreats of the ice within the glacial period. During each 

 retreat the melting of the ice freed rock waste, which had been 

 carried aloag beneath the ice or frozen into the glacier, and 

 deposited this waste as a sheet of till over all the land uncovered 

 by the melting away of the ice. One of the oldest of these till- 

 sheets is the Illinoian, now exposed over most of the southern and 

 western parts of the state, but concealed by subsequent sheets 

 in the northeast, the most extensive of which is the Wisconsin, 

 In the southern two-thirds of the state the Illinoian and Wisconsin 

 sheets are apparently the only ones represented, and their com- 

 mon boundary is the great terminal moraine, built during a pro- 

 longed halt of the ice-front at its farthest advance in the 

 Wisconsin stage, and called here the Shelbyville moraine. The 

 time between these two ice invasions, represented by the two 

 drift layers, corresponds with two or more stages represented 

 elsewhere by differentiable till-sheets, but known here only by 

 erosion and weathering in the older drift. In other words, in 

 southern Illinois, we have an old till-sheet extending far south, 

 the Illinoian ; a much younger till-sheet extending southward 

 about to Paris, Mattoon, and Shelbyville, the early Wisconsin; 

 and a strong terminal moraine lying along their common boun- 

 dary. Fig. I shows the distribution of these features. For 



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